Field of the Invention
The present invention concerns a method, a magnetic resonance (MR) apparatus, and a non-transitory computer-readable data storage medium for determining a shim setting in order to increase the homogeneity of a basic magnetic field of a magnetic resonance apparatus, by operation of a shim system.
Description of the Prior Art
In the MR data acquisition scanner of a magnetic resonance apparatus, the body of an examination subject, in particular a patient that is to be examined is typically exposed to a relatively high basic magnetic field, 1.5 or 3 or 7 tesla for example, with the use of a basic magnet. In addition, gradient pulses are applied with the use of a gradient coil arrangement. Radio-frequency pulses, for example excitation pulses, are then transmitted via a radio-frequency antenna arrangement by suitable antennas, resulting in the nuclear spins of specific atoms of the subject which are excited into resonance by the radio-frequency pulses, being tipped through a defined flip angle with respect to the field lines of the basic magnetic field. During the relaxation of these nuclear spins, radio-frequency signals, also known as magnetic resonance signals, are emitted, and are received by suitable radio-frequency antennas and then processed further. The desired image data can be reconstructed from the raw data acquired in this way.
Accordingly, it is necessary to transmit a specific magnetic resonance sequence, also known as a pulse sequence, for a specific measurement. This magnetic resonance sequence is composed of a train of radio-frequency pulses, excitation pulses and refocusing pulses for example, as well as, appropriately times, gradient pulses that are transmitted in a coordinated manner in different gradient axes along different spatial directions. Readout windows, matched with respect to time to the pulses, are set that specify the time periods in which the induced magnetic resonance signals are captured.
In magnetic resonance imaging with such a magnetic resonance apparatus, the homogeneity of the basic magnetic field in the examination volume is a factor of major importance. Even small deviations in homogeneity can lead to large deviations in a frequency distribution of the nuclear spins, with the result that qualitatively inferior magnetic resonance image data are acquired.
Shim systems are known for the purpose of improving the homogeneity in the examination volume. Although shim plates of ferromagnetic material can be strategically placed at locations so as to influence this homogeneity, the shim systems of concern herein are conductive shim elements provided with currents by a shim supply such as a processor-controlled amplifier. When a magnetic resonance apparatus is installed at its intended deployment site, fields present in the environment can limit the homogeneity of the basic magnetic field, in particular around an isocenter of the magnetic resonance scanner. For that reason, when a magnetic resonance apparatus is installed and commissioned, often in conjunction with calibration measurements, the shim element is adjusted so as to establish a maximally optimal homogeneity. Accordingly, basic shim settings are calculated at the time of installation and commissioning of the magnetic resonance apparatus.
However, the examination subject that is to be scanned itself constitutes a further source of inhomogeneity. For example, when a person who is to be examined is introduced into the magnetic resonance scanner, the material of the body causes further disruption to the homogeneity. In order to counteract this problem, it is known to use an adjustable shim system. In particular, shim systems are known for this purpose that, when the shim elements thereof are driven by means of different shim currents, generate different compensation magnetic fields in order to improve the homogeneity.
The shim elements of the adjustable shim system can be integrated into the magnetic resonance scanner and be enclosed by the housing thereof or may be integrated into a local coil assembly that is used to transmit and/or receive the aforementioned radio-frequency signals. Local coils can be positioned on the surface of a patient's body or at a short distance therefrom, for example at up to 5 cm. Typical local coils are reception coils for detecting the magnetic resonance signals. There are also local coils that are designed to transmit radio-frequency pulses. Coils of this type are also referred to as local transmit coils or as local transmit/reception coils, and may include an adjustable shim element. Typically, an adjustable shim element integrated into a local coil is used in addition to another adjustable shim element enclosed by the housing of the magnetic resonance scanner.
In order to shim the disruptions of the subject that is to be examined, it is conventional practice, when initially driving the shim coil or coils according to basic shim settings obtained during the installation and commissioning of the magnetic resonance scanner, to use the magnetic resonance apparatus itself to perform a measurement of the field distribution when the person to be examined has been introduced into a patient receiving zone of the magnetic resonance scanner. Thereafter, using the basic shim settings as a starting point, optimized shim settings are determined by a computer while taking into account the measured field distribution. Using the optimized shim settings, the shim element or elements is/are then driven in order to achieve a maximally optimal homogeneity. Typically, basic shim settings of the type exist for shim systems having shim elements that are not integrated into local coils.
Shim systems with coils that are integrated in local coils in most cases generate local and/or spatially strongly varying compensation fields, which are subject to a strong spatial modulation. Minor spatial changes to such shim elements, for example displacements by a few millimeters, and/or small deviations between an applied and a realized shim setting, in some cases cause such differences between a desired and a generated compensation field that the homogeneity of the basic magnetic field may be increased to a lesser degree than expected or, as the case may be, even be made worse. Consequently, a precise determination of the shim setting, particularly in the case of shim coils that are integrated in local coils, is essential.